Estudo da reação de Simmons-Smith : obtenção vinilciclopropanos substituidos de interesse industrial

AUTOR(ES)
DATA DE PUBLICAÇÃO

1990

RESUMO

Of the procedures described in the literature, that of Rawson e Harrison gives under standard conditions the best results for the cyclopropanation of cyclohexene. We found that the noble metal salt used to activate the metallic zinc has a more important role than to clean its surface, as the yield of the reaction is strongly influenced by the anion. In the presence of 2% cuprous chloride as an activator we obtained the best yield (90%) of byciclo[4.1.0]heptane. Electron paramagnetic ressonance measurements have shown that the copper sant is not reduced. We believe that it acts as a catalyst for the carbene transfer from the organozinc intermediate to the olefin. Use of ultrasonic irradiation accelerates the reaction, principally if methylene iodide is used, giving byciclo[4.1.0]heptane in 88% yield after only 6 h of total reaction time. Zinc dispersed on graphite and zinc obtained by the Rieke´s method, which are both very reactive, are shown to be inadequate for the substitution of the convencional zinc/copper couple. The general procedure developed was used for the cyclopropanation of 1,3-dienes, which are shown to be less reactive because of the delocalization of its p electrons. The conversion of butadiene is 18% and vinylcyclopropane is obtained with 68% selectivity. The same procedure applied to the reaction of 2,5-dimethyl-2,4-hexadiene with methyl dibromoacetate allows the preparation of the pyrethroid methyl crysanthemate with 13% yield. On the other hand, the procedure fails in the cyclopropanation of the methyl ester of 2,4-hexadienoic acid (methyl sorbate) because of the reduced p electron density due to the conjugation with the carboxylic group.

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